History of Biotic Pollination

The evolutionary histories of the most important plants in our lives, the flowering plants or angiosperms, are relatively unknown. Today, angiosperms are almost all pollinated by animals, the most important being the insects. Some of the largest plant families are very well adapted for insect pollination (for example the Orchidaceae)1. Insect pollination not only conserves energy for the plant (less pollen/ ovule produced), but insects may play a part in angiosperm diversity2. As important to angiosperms as insects are, the plants are in turn valuable to the pollinator. Insects derive food, shelter, and breeding places from the plants they visit. Adaptations made by both angiosperms and insects to exploit the offering of the other have lead to coevolution. Analyzing the way insects and plants interact today can offer only a tiny picture into the evolution of angiosperms. Examining fossils give us a better idea of the pollination mechanisms already in place in the angiosperms during their evolution2,3.

Fossil evidence of non-angiospermous plants suggests that pollination by insects was in place well before the rising of the angiosperms3. Early insects were well adapted for consumption of plant reproductive parts. It is possible that while feeding on some pollen or ovules, an insect accidentally pollinated an ancient plant, and could have been the very first step to the evolution of the angiosperms4. All four groups of pollinating insects evolved well before angiosperms: Coleoptera (beetles) were very diverse during the Upper Carboniferous, while the Diptera (flies) and the Hymenoptera (bees and wasps), very important pollinators today, appeared during the Triassic. Lepidoptera (moths and butterflies), also important pollinators, do not show up in the fossil record until the Lower Cretaceous2. The earliest Cretaceous flowers were probably small magnoliid types with few parts, where the only pollinator reward was most likely pollen. "Generalist" insects probably pollinated these flowers: beetles, short-tongued wasps and flies5. By the Middle Cretaceous, representatives from the Magnoliidae, Hamamelidae, and Dilleniidae appear. Several structures occur in representatives of these angiosperms that suggest insect pollination (nectaries, petals, staminodes, etc.)6. Rosids with characters such as well-developed corollas, receptacular areas, and nectaries were established during the Middle Cretaceous, and by the Late Cretaceous, were very diverse7,8. At this time the Hymenoptera and Diptera were also going through radiations, as more "evolved" types (long-tongued bees and flies, etc.) show up in the fossil record5. The more derived floral types of the asterids didn't appear until the Tertiary and coincides with the appearance of stingless honeybees in the Late Cretaceous9.

The fossils of the New Jersey Raritan Formation are very diverse, with hundreds of species of plants from mosses to angiosperms. The fossil flowers alone represent over 200 species from several different families of angiosperms. While there are "primitive" representatives (i.e. magnoliid and nymphaeacious flowers) in this formation, there are also more "advanced" representatives. These flowers contain characteristics suggesting "advanced" insect pollinators from possible reward substances (see Paleoclusia) to viscen thread on pollen (see Paleoenkianthus). Such advanced pollinators, as well as other insects, have been found encased in amber in this formation10,11,12. As the earth gives up more of it's fossil treasures, we will eventually gain a better understanding of angiosperm/insect coevolution.